1. Field of the Invention
The present invention relates to a method of forming a contact hole and a method of manufacturing a semiconductor device. More specifically, the present invention relates to a method of forming a contact hole and a method of manufacturing a semiconductor device each including a step of adjusting an opening size of contact hole patterns formed in a resist film.
2. Related Background Art
In recent manufacturing of a semiconductor device a scale-down of which is advanced, not only a wavelength of an exposure light used to form a resist pattern at a lithography step is made shorter, but also a certain processing is additionally performed after forming the resist pattern, thereby adjusting a size of the resist pattern. For example, to obtain a very small contact hole, a hole diameter (a hole size) of a contact hole pattern formed in the resist film is often reduced.
Among specific examples of a process for reducing the hole diameter, a reflow process is most popular. In the reflow process, a semiconductor substrate is subjected to a heat treatment after contact hole patterns are formed in a resist film, thereby causing the resist film to slightly flow into holes and reducing the hole diameter, accordingly.
As the process for reducing the hole diameter, there is also known a so-called RELACS® (Resolution Enhancement Lithography Assisted by Chemical Shrink) process. In this RELACS® process, a water-soluble resin, which has a crosslinking reaction with an acid component in a resist film when being heated, is spin-coated on a semiconductor substrate after contact hole patterns are formed. Thereafter, a heat treatment triggers the crosslinking reaction between the water-soluble resin and the acid component in the resist film, thereby forming a film on a surface of the resist film and reducing the hole diameter, accordingly. The unnecessary water-soluble resin film is removed by a rinse solution.
Further, as the process for reducing the hole diameter, there is known a so-called SAFIER® process. In the SAFIER® process, a water-soluble resin, which shrinks when being heated, is spin-coated on a semiconductor substrate after forming contact hole patterns. The semiconductor substrate is then heated to extend a resist film in a direction parallel to a surface of the substrate by a shrink force of the water-soluble resin, thereby reducing the hole diameter.
By using one of these processes, very small contact holes which cannot be formed only by ordinary process can be formed.
Meanwhile, as one type of a photomask, there is known a photomask which has one opening pattern corresponding to one contact hole, and which has a dense part having opening patterns arranged at high density and a less-dense part having opening patterns arranged at low density. If contact hole patterns are to be formed on a resist film using such a photomask, it is conventionally, disadvantageously difficult to obtain contact hole patterns at an equal size in the dense part and the less-dense part. This conventional disadvantage will be described in more detail.
Normally, if a pattern density of the photomask is lower, a focus margin is lower. Therefore, to secure a sufficient focus margin in the less-dense part in which contact hole patterns are arranged at low density, the size of each opening pattern in the less-dense part is set larger than that in the dense part. If the size of the opening pattern in the less-dense part is set larger, the size of the contact hole pattern to be formed is, quite naturally, larger. Due to this, when the hole diameter of each contact hole pattern formed in the resist film is reduced by using the above process, it is necessary to make a variation of the hole diameter large in the less-dense part and small in the dense part so as to make the hole diameter in the dense part equal to that in the less-dense part. In other words, it is required, in the process of adjusting the hole diameter of each contact hole pattern, to change the variation of the hole diameter depending on whether the patterns are arranged at high density or low density.
Among the above-stated processes, the RELACS® process and the SAFIER® process have no variation of the hole diameter depending on whether the patterns arrangement is dense or less dense. In practice, therefore, it is difficult to use these processes for the hole formation method. Namely, neither the RELACS® process nor the SAFIER® process can solve the disadvantage that the focus margin differs between the dense part and the less-dense part of the photomask.
The reflow process, by contrast, is characterized in that the variation of the hole diameter in the less-dense part is larger than that in the dense part. Therefore, by forming the photomask while considering the difference in the variation of the hole diameter between the dense part and the less-dense part of the photomask, it is possible to form contact hole patterns at high density and contact hole patterns at a low density with an equal size. Namely, this reflow process can solve the disadvantage that the focus margin differs between the dense part and the less-dense part of the photomask.
Nevertheless, the reflow process is confronted with a disadvantage that the variation of the hole diameter is greatly influenced by accuracy of a heat process in the less-dense in comparison with the dense part. This derives from the fact that the hole size is larger in the less-dense part than in the dense part. In addition, if process conditions such as resist conditions (e.g., a resist material and a resist film thickness) and heating conditions are changed, the variation of the hole diameter is changed. It is disadvantageously necessary, therefore, to correct the photomask according to the opening pattern density.